Seed priming or osmoconditioning are terms to describe a pre-sowing hydration treatment developed to improve seedling establishment (Heydecker and Coolbear, 1977, Seed Science and Technology 5:353-425), (Khan et al., 1980/81, Israel J. Bot. 29:133-144) and (Bradford, 1986, HortScience 21:1105-1112). The general procedure is described as follows: Seeds are placed in an aerated osmotic solution of known water potential (.psi.). Different osmoticums have been used to achieve the solution .psi. for priming including high molecular weight (MW of 6000-8000) polyethylene glycol (PEG) and/or inorganic salts (Heydecker and Coolbear, 1977, supra). The osmotic solutions induce a water stress which prevents the completion of germination (radicle emergence). Seeds are, however, able to imbibe and complete the early phases of germination under controlled conditions. The duration of the priming treatment has been reported to be from less than 24 hours (Guedes and Cantliffe, 1980, J. Amer. Soc. Hort. Sci. 105:777-781) to several weeks (Khan et al., 1980/81, supra) and is dependant on species, cultivar and even seedlot. Seeds are then removed from the solutions, rinsed and then dried back. After priming, seeds can be sown with existing planters. Results obtained from priming different crop seeds have been reviewed by Bradford, 1986, supra. In general, seedling emergence of primed seeds is earlier and more sychronous than dry sown seeds. Improvements in plant growth and yield characteristics have also been documented.
Several technical and logistical difficulties have been encountered with priming. Osmotic solutions require continuous aeration and in general, a large volume of priming solution is required per quantity of seeds. Reductions in percentage seedling emergence have been reported with the use of inorganic salts (Brocklehurst and Dearman, 1984, Ann. Applied Biol. 102:585-593). A relatively high concentration of PEG in solution is viscous and has low oxygen solubility and diffusivity (Mexal et al., 1975, Plant Physiol. 55:20-24).
An alternative to the use of solutions is to prime seeds in a solid medium. A solid material should have a high water holding capacity and allow seeds to imbibe. Peterson (1976, Sci. Hort. 5:207-214) described a procedure to prime onion seeds in a slurry of PEG 6000 and vermiculite. A ratio of three to four parts vermiculite moistened with PEG solution were mixed with 1 part seed. Although this technique overcame the need for aeration, he reported that the seeds were difficult to separate from the vermiculite. Methods have been described to increase the seed moisture content of large-seeded vegetables prior to sowing (Bennett and Waters, 1984, J. Amer. Soc. Hort. Sci. 109: 623-626 and 1987, J. Amer. Soc. Hort. Sci., 112:45-49). Seed moisture content of lima beans (Phaseolus lunatus) and sweet corn (Zea mays) was increased by combining seed, vermiculite and a known amount of water in a closed container. This mixture was incubated and then the vermiculite sieved away prior to sowing. Seedling establishment was improved by the pre-sowing treatments for both crops.
The present invention describes the use of specific solid phase media to prime small seeded vegetables. The specific solid matrix provided a structure to hold water and allow gas exchange. Water relations of the solid material-seed mixture were characterized and seedling emergence studies were conducted on tomato, carrot and onion. Alleviation of thermodormancy by solid matrix priming was studied in lettuce.